Polysuccinimide ("PSI", also referred to as "anhydropolyaspartic acid" or "polyanhydroaspartic acid") may be prepared by the thermal polycondensation of L-aspartic acid. This polymer is then typically base hydrolyzed to form polyaspartate salt ("PAS") which has industrial uses such as scale and corrosion inhibitors and as a lubricant in a metal working fluid(s).
Crosslinking PSI and then converting PSI to its corresponding polymer salt gives its corresponding polymer salt a much desired superabsorbent property (hereinafter such polymer salts are referred to as superabsorbent polymers or "SAP") This property is highly desired since the crosslinked PSI can absorb many times its own weight of a liquid (such as water) thereby creating another important utility for use in diapers and the like. Over seventy different kinds of diapers are sold today in a $3.6 billion-a-year United States disposable diaper market for infants and for persons who suffer from stress urinary incontinence, a debilitating medical condition. These superabsorbent polymers are used as sanitary materials for other hygienic articles, for agricultural materials such as water-holding materials for soil and sheets for seedlings, food-related material/s such as food-freshness-keeping agents and dehydrating agents, public works materials or building materials such as dew inhibiting sheets on buildings and water sealing agents, and the like.
These superabsorbent polymers of the prior art which are prepared by initially crosslinking polymers which are typically not biodegradable. They tend to be environmentally durable and can exist in water or soil semi-permanently. Such durability is a desirable property for some uses; however, in others it might raise a concern at times in view of undesired environmental preservation after disposal because durable materials are not readily biodegraded to other biodegradable.
There have been several technical reports and disclosures disclosing a technique(s) for preparing water-insoluble superabsorbent polymers by crosslinking polyamino acids which have excellent safety and degradability.
Use of crosslinked PSI as an absorber is disclosed in U.S. Pat. No. 5,525,703 ('703) to Dennis J. Kalota, issued on Jun. 11, 1996, wherein the crosslinking agent is a specific polyamine. In that '703 patent and in U.S. Pat. No. 5,525,682 to Nagatomo et al., and in U.S. Pat. No. 5,461,085 to Nagatomo et al., which issued Oct. 24, 1995, crosslinking is carried out in solution in an organic solvent such as dimethyl formamide or while suspended in water. To isolate the crosslinked polymer, the water must be removed or the polymer precipitated and the solvent removed.
Prior art technical literature (G. Giammona et al., Synthesis and characterization of water swellable .alpha., .beta.-polyasparthydrazide derivatives, 272 COLLOID & POLYMER SCIENCE 1637-1641 (1994); G. Giammona et al., Crosslinked .alpha., .beta.-Polyasparthydrazide Micromatrices for Controlled Release an Anticancer Drugs, 10 JOURNAL OF BIOACTIVE AND COMPATIBLE POLYMERS 28-40 (1995); and G. Giammona, 41 JOURNAL OF CONTROLLED RELEASE 195-203 (1996)) discloses that a water-swellable polymer can be made from partially crosslinking polyasparthydrazide and glutaraldehyde producing a crosslinked polyhydrazide product. The polyasparthydrazide is made by reacting polysuccinimide with equal moles of hydrazine. All the PSI units are converted to hydrazide with the hydrazine and then the hydrazides are partially bridged with glutaraldehyde. However, the instant invention is unique from the art in that the superabsorbent polymer of this invention is a crosslinked PAS (sodium) salt and is not a crosslinked polyhydrazide as the prior art literature discloses.
Water absorbency by such a crosslinked polyhydrazide, which is typical of the prior art, is quite low.